DEVELOPMENT OP CONNECTIVE SUBSTANCE AND BLOOD. 19T 



migrates in between the primary germ-layers, and becomes blood and connec- 

 tive substance." 



According to the recent investigations of Babl, Zieslbr, van Wijhb,. 

 EiJCKBET, and others, the mesenchyme is produced froni various regions of 

 the middle germ-layer. A participation of the inner germ-layer in the forma- 

 tion of the blood-vessels is rendered probable. 



Summary. 



1. Besides the four germ-layers, which have the form of 

 epithelial lamellsB, special germs are developed in the higher 

 "Vertebrates for the sustentative substances and the blood, — the 

 mesenchyme-germs. The latter together make up the intermediate- 

 layer. 



2. The mesenchyme-germs arise by cells detaching themselves 

 from epithelial union with the germ-layers, and penetrating a& 

 migratory cells into the fissure between the four germ-layers (the- 

 remnant of the original cleavage-cavity) and spreading themselves out 

 in this space. 



3. Germ-layers and mesenchyme -germ (intermediate layer) ex- 

 uibit a difference in the method of their origin : the former are- 

 developed by foldings of the wall of the blastula, the latter by emi- 

 gration of isolated cells from definite territories of the germ-layers. 



4. Mesenchyme-germs arise from the wall of the primitive segment, 

 from the cutis-plate, and at certain regions of the parietal and' 

 visceral lamellte of the middle germ-layer. 



5. Blood-vessels are developed both in the body of the embryo- 

 itself, in a manner which still remains to be accurately determined, 

 and also in the territory of the area opaca of meroblastic eggs. 



6. The source of the cells from which the vessels and blood of 

 the opaque area arise is at present a matter of controversy. 



7. In the formation of vessels in the opaque area the following 

 phenomena are to be regarded : — 



(a) The embryonic cells of the intermediate layer arrange- 

 themselves : — 



First into a network of cords, and 

 Secondly into the substance-islands (Substanzinseln). 

 (6) There are developed out of the cell-cords, at the same time 

 with the secretion of the fluid portions of the blood, the 

 endothelial wall of the primitive blood-vessels and their 

 cellular contents, the blood-corpuscles (blood-islands), 

 (c) The Substanzinseln become embryonic connective substance 



